Because nitride semiconductor materials have sufficiently wide forbidden bands with direct interband transition, their applications to short-wavelength light-emitting devices have been widely investigated. In addition, because of a large saturated drift velocity of electrons and the usability of a two-dimensional carrier gas in heterojunctions, their applications to electron devices are expected.
Nitride semiconductor layers for constituting these devices are obtained by epitaxial growth on base substrates by vapor phase growth methods such as a metal-organic vapor-phase epitaxy method (MOVPE), a molecular beam epitaxy method (MBE), a hydride vapor-phase epitaxy method (HVPE), etc. However, because there are no base substrates having lattice constants matched to those of the nitride semiconductor layers, it is difficult to grow high-quality layers, resulting in nitride semiconductor layers with a lot of crystal defects. Because the crystal defects hinder devices from having improved characteristics, investigation has been vigorously carried out so far to reduce the crystal defects in the nitride semiconductor layers.
Known as a method for producing group-III nitride crystals with relatively few crystal defects is a method in which a low-temperature-deposited layer (buffer layer) is formed on a different substrate such as sapphire, etc., and an epitaxially grown layer is formed thereon. In a crystal-growing method using a low-temperature-deposited layer, AlN or GaN is deposited on a substrate of sapphire, etc. at about 500° C., and an amorphous or partially polycrystalline, continuous film is formed. The film is heated to about 1000° C. to cause partial evaporation or crystallization, thereby forming high-density crystal nuclei, which are used as growth nuclei to obtain a GaN film with relatively good crystallinity. Even by the method of forming the low-temperature-deposited layer, however, the resultant substrates are filled with considerable numbers of crystal defects such as threading dislocations, vacancies, etc., insufficient for obtaining presently demanded high-performance devices.
In view of the above circumstances, investigation has been intensively carried out to obtain methods of using a GaN substrate as a substrate, from which a crystal grows, and forming a multi-layered semiconductor film, on which devices are formed, on the GaN substrate. The GaN substrate for crystal growth is referred to herein as a self-supported GaN substrate. The epitaxial lateral overgrowth (ELO) technology is known as a method for forming a self-supported GaN substrate. The ELO method is a technology of forming a GaN layer with few dislocations by laterally growing the GaN layer on a base substrate in openings of a mask formed thereon. JP 11-251253 A discloses the production of a self-supported GaN substrate by forming a GaN layer on a sapphire substrate by this ELO method and then removing the sapphire substrate by etching, etc.
A facet-initiated epitaxial lateral overgrowth (FIELO) method (A. Usui, et al., Jpn. J. Appl. Phys. Vol. 36 (1997) pp. L.899-L.902) has evolved from the ELO method. The FIELO method is different from the ELO method in forming facets in openings of a silicon oxide mask in the selective growth of GaN. The facets change the propagation direction of dislocations, thereby reducing the number of threading dislocations reaching the top surface of the epitaxially grown layer. The growth of a thick GaN layer on a base substrate of sapphire, etc. by the FIELO method and the removal of the base substrate thereafter can provide a high-quality, self-supported GaN substrate with relatively few crystal defects.
As a method for forming a low-dislocation, self-supported GaN substrate, a method of dislocation elimination by the epi-growth with inverted-pyramidal pits (DEEP method) has been developed (K. Motoki et. al., Jpn. J. Appl. Phys. Vol. 40, JP 2003-165799 A). The DEEP method intentionally forms a plurality of pits surrounded by facet planes on a crystal surface by growing GaN with a mask of silicon nitride, etc. patterned on a GaAs substrate, and concentrates dislocations at the bottom of the pits to lower a dislocation density in other regions.
In an as-grown state, the GaN substrate obtained by the ELO method or the DEEP method usually has morphology such as pits, hillocks, etc. on its surface, resulting in difficulty in growing an epitaxial layer for producing devices without further treatment. Therefore, the top surface of the substrate is generally mirror-polished before devices are produced thereon.
In such circumstances, JP 2003-178984 A proposes a method for producing a group-III nitride semiconductor substrate having a low dislocation density, which comprises the steps of forming a metal film on a base substrate composed of a sapphire substrate and a first group-III nitride semiconductor layer deposited thereon, or on a base substrate made of a first group-III nitride semiconductor; heat-treating the base substrate in an atmosphere comprising a hydrogen gas or a hydrogen-containing compound gas to form voids in the first group-III nitride semiconductor layer; and forming a second group-III nitride semiconductor layer on the metal film. JP 2003-178984 A describes in Example 14 and FIG. 16 a self-supported GaN substrate, whose fluorescent photomicrograph in the cross section shows no black stripes but substantially uniform black portions near an peeling interface with the sapphire substrate. With respect to this phenomenon, JP 2003-178984 A describes that increase in the amount of hydrogen in a carrier gas suppresses defects from growing to the surface.
It has been found, however, that the self-supported GaN substrate produced by such a method has a nonuniform carrier concentration on its surface despite the lowered dislocation density. The nonuniform carrier concentration distribution on its substrate surface is a problem that has never occurred in conventionally used semiconductor substrates of Si and GaAs because of their production methods. In the self-supported GaN substrate, however, there may be locally nonuniform regions in the carrier concentration because an epitaxially grown thick crystal layer is used as the substrate. When crystal growth is carried out while forming facets in a growth interface, to lower the dislocation of the self-supported GaN substrate, there inevitably occurs difference between facet planes and other planes, resulting in different rates of crystal growth and thus differences in effective segregation coefficients of impurities therebetween, which leads to the nonuniform impurity distributions, namely variations in the carrier concentrations. Because regions with different carrier concentrations appear as the hysteresis of facet-grown regions, they are distributed such that they extend in a crystal growth direction. If the regions with different carrier concentrations reached the top surface of the substrate, variations in the carrier concentration would inevitably occur on the top surface of the substrate.
It has been found that when there are regions having nonuniform carrier concentrations on the surface of the GaN substrate, epitaxial GaN layers grown on such regions are prone to have large surface roughness. Namely, even if an underlying GaN substrate is mirror-polished, there occurs a phenomenon that the resultant epitaxial layer has a rough surface. Without an epitaxial GaN layer having a uniform surface morphology, the characteristics of devices formed thereon would suffer from deterioration, variations, etc.
When a crystal grows while forming pits surrounded by facet planes in crystal growth interfaces, dislocations are concentrated in the bottoms of the pits. All accumulated dislocations are not necessarily integrated, but form high-dislocation regions expanding without clear boundaries. In regions in which dislocations are concentrated without clear boundaries, it is considered that regions locally having nonuniform carrier concentration distributions are formed by the diffusion of impurities.
Even in GaN crystals, in which the concentrations of dislocations in the bottoms of the pits are suppressed, nonuniform carrier concentration distributions may occur on their surfaces. If epitaxial GaN layers were caused to grow on such GaN crystal substrates, ragged morphologies would appear on their surfaces, whose roughness is not substantially different from that of GaN substrates with dislocation-concentrated regions. This suggests that the roughness of the epitaxial surface is caused not by a dislocation density distribution but by a local distribution of the carrier concentration.
If the growth of facets were terminated by increasing the amount of hydrogen in a carrier gas as in JP 2003-178984 A, or by changing crystal growth conditions in the course of crystal growth, it might be considered that a crystal growth interface becomes flat, resulting in a uniform surface carrier concentration distribution. However, because there has never conventionally been a concept of substantially uniformly controlling the carrier concentration distribution on the top surface of the substrate, the polishing of a substrate surface removes even regions having a uniform carrier concentration distribution, resulting in the likelihood that there are large variations in the carrier concentration on the mirror-finished substrate surface. No investigation has conventionally been conducted as to how thick a surface layer having a uniform carrier concentration distribution should be. Accordingly, even if a GaN substrate with a surface layer having a uniform carrier concentration distribution were produced, the mirror-finishing treatment would likely remove almost the entire surface layer or make it too thin. It has thus been impossible to stably produce a low-dislocation GaN substrate having small variations of a carrier concentration on the surface and providing devices formed thereon with no defects.